Upper Air Sounding Details

Upper air soundings are plotted data from balloons. The
balloon records temperature, humidity and winds and these are plotted versus
pressure/height to give details on the vertical structure of the atmosphere.
The type of plot is called a SkewT and a set of SkewTs are available for
most US stations. These are updated once every 12 hours at roughly 10:00
EST.

SkewT Plots

Sample SkewT plot

A Skew T plot is a standard plot used by meteorologists
to analyze data from a balloon sounding. This is a plot of temperature with
height as denoted by pressure.

The pressure lines are plotted horizontally in blue and are also on an
inverse log scale. The concept of Skew T means that the temperature is
not plotted vertically but angles off to the right at a 45 degree angle.
The temperature lines of the Skew T are in blue. The green lines are called
dry adiabats. The light blue dashed lines are saturation adiabats. The
yellow dashed lines are lines of constant mixing ratio.

The sounding is plotted as two white lines. The right
line is the temperature profile. The left line is the dewpoint profile.

The winds are plotted as wind barbs with height (see
below) on the right edge of the plot.

The sounding also has height information plotted at the
left, next to the pressure:

Specific sounding related parameters are plotted in the
upper right. These values are:

TP: Tropopause level (mb)

MW: Max wind level (mb)

FRZ: Lowest freezing level (mb) or BG for below ground

WB0: Wet bulb zero (mb) or BG

PW: Precipitable water (in)

RH: Mean RH surface to 500 mb (%)

MAXT: Estimated max temperature (C) using a 150mb layer

TH: 1000-500mb thickness (m)

L57: 700-500mb lapse rate (C/km)

LCL: Lift condensation level (mb) from surface data

LI: Lifted index (C) using 100 mean layer above surface

SI: Showalter index (C)

TT: Total totals index

KI: K index

SW: Sweat index

EI: Energy index

-PARCEL-
This is a parcel trajectory (the yellow line on the sounding) based
on 100 mb mean layer.

Sounding diagrams provide a important means for determining
the stability of the atmosphere above a specific location. By using the
concept of an air parcel, lifting it or lowering it and comparing the
resulting parcel conditions to the conditions of the surrounding environment
as defined by the balloon sounding.

Parcels of unsaturated air tend to follow the dry adiabat
lines (green lines) as they ascend or descend. The saturation adiabats (light
blue) lines show how parcels saturated with water vapor will ascend or
descend. Descending parcels will tend to unsaturate immediately. The mixing
ratio lines (yellow) relate to the amount of water vapor in a parcel in grams
of water vapor per kilograms of dry air. Parcels of air attempt to maintain
a constant mixing ratio as they ascend or descend. Generally, a parcel will
rise, following the dry adiabat until it saturates. This occurs when dry
adiabat crosses the initial mixing ratio line. This is considered the LCL.
If lifting continues, the parcel cools following the saturation adiabat. If
the parcel descends, it will always follow the dry adiabat as it will
immediately unsaturate if saturated. By then comparing the parcel temperature
to the environment, you can determine whether it is stable (parcel cooler) or
unstable (parcel warmer). An unstable parcel will accelerate upwards and is
the primary means for thunderstorm development. A stable parcel will
decelerate and eventually descend. This is the typical atmospheric condition
and it the primary condition in high pressure areas. The descending air
desaturates the atmosphere and leads to clearing skies and calm conditions.